This invention relates to the manufacture of piezoelectric material and, more particularly to the manufacture of piezoelectric material by molecularly polarizing a polymer material.
As is known in the art, certain polymer and copolymer materials may be provided with piezoelectric properties. For example, a sheet of a polymer material comprising polyvinyldene fluoride (PVDF) having long repeating chains of CH.sub.2 --CF.sub.2 molecules may have these chains "orientated" to provide a net polarization. Such a sheet of orientated material disposed between a pair of electrodes, for example, may be used to either detect or produce mechanical energy. In one mode of operation, mechanical energy incident upon the combination of electrodes and sheet of material will mechanically deform the orientated crystalline structure of the sheet. This mechanical change results in a voltage potential being produced through the sheet of material and, hence, provides a potential difference between the pair of electrodes. In a second mode of operation, a voltage applied to the pair of electrodes induces a potential difference through the sheet of material causing the orientated structure of the material and hence the combination to mechanically deform in response to the voltage difference, thereby producing mechanical energy.
Several methods are known in the art for providing very thin films of molecularly polarized polymer materials such as PVDF material having suitable piezoelectric properties. One such method described in a paper entitled "Piezoelectricity Of Corona-Poled Poly (vinylidene fluoride)" by T. Furokawa et al, The Society of Polymer Science, Japan, Vol. 36, No. 10, pps. 685--688, Oct. 1979. In the described method, the ends of a sheet of PVDF material are secured between a pair of members of a stretching apparatus. Both of these members move away from each other to stretch the material. Prior to stretching the material, however, the apparatus is placed within an oven which is heated to a predetermined temperature until the material of the sheet softens. At this juncture, the material is stretched by moving both securing members away from each other. While the material is being stretched, corona discharge is provided above and below the film to create an electric field across the film to orientate the PVDF material. While this technique produces polarized material having piezoelectric properties, still there are several problems associated with this technique. The first problem is that by heating the entire film, it is difficult to keep the ends of the sheet locked in the members of the stretching apparatus. This is because the entire sheet softens including the ends of the sheet disposed within the members. These ends may have a tendency while the sheet is being stretched to slip out of the members thereby aborting the polarizing and stretching run. Since the temperature of the sheet cannot readily be changed and the force applied to the sheet also cannot be changed during the process, the stretch ratio of the sheet is thus generally limited to the so-called "natural draw ratio" of the plastic which here for PVDF is about 4:1. Higher stretch ratios should provide material having better piezoelectric properties. Furthermore, with this technique, it would be difficult to stretch and pole a relatively large sheet since the oven would have to be made correspondingly large. Since each sheet, however large, has to be loaded within the oven and the oven then brought up to operating temperature, this technique would also be difficult to adapt for a continuous stretching and poling operation. Therefore, this technique is not well suited for stretching and poling large quantities or continuous runs of PVDF material.